Use latest juju 2.1 branch for snap builds until GA
Use latest juju 2.1 branch for snap builds until GA

We want to continually provide edge snap builds using the latest juju 2.1 code
to work out any potential blocker bugs.

Signed-off-by: Adam Stokes <49c255c1d074742f60d19fdba5e2aa5a34add567@users.noreply.github.com>

Ubuntu-Solutions-Engineering/conjure,Ubuntu-Solutions-Engineering/conjure,ubuntu/conjure-up,conjure-up/conjure-up,ubuntu/conjure-up,conjure-up/conjure-up